Shifting from one speed ratio to another in an automatic transmission is generally initiated based on load (as judged by engine throttle position) and engine or transmission output speed. In an electronic control, speed and load dependent thresholds (referred to collectively as a shift pattern) are stored in a look-up table, and the actual speed and load are periodically determined and compared to the stored thresholds to determine if a shift should be initiated. To provide hysteresis, separate thresholds are provided for upshifting and downshifting, with an upshift being initiated when the speed/load point rises above the upshift threshold, and a downshift being initiated when the speed/load point falls below the downshift threshold.
An example of a shift pattern is given by the solid traces in FIG. 3, where the trace 80 represents an upshift threshold for upshifting from 1.sup.st gear to 2.sup.nd gear, and the trace 82 represents a downshift threshold for downshifting from 2.sup.nd gear to 1.sup.st gear. When 1.sup.st gear is engaged, the transmission controller can look-up a 1-2 speed threshold based on engine throttle position using trace 80, and initiate an upshift to 2.sup.nd gear if and when the measured speed output speed exceeds the 1-2 speed threshold. Similarly, when 2.sup.nd gear is engaged, the controller can look up a 2-1 speed threshold based on throttle position using trace 82, and initiate a downshift to 1.sup.st gear if and when the measured output speed falls below the 2-1 speed threshold.
The stored shift pattern directly impacts vehicle performance, engine fuel economy and driveline noise, and the various thresholds are calibrated to reasonably satisfy these criteria under typical driving conditions, as much as possible. In electronically controlled transmissions, additional flexibility can be achieved by providing two or more different shift patterns, which can be selected by the driver depending on operating conditions (hilly terrain, for example) or simply driver preference. For example, in a "performance" setting, the traces 80 and 82 of FIG. 3 can be shifted up somewhat so that both upshifting and downshifting occur at higher speeds, for a given throttle setting. From the driver's viewpoint, this delays shifting to a higher gear, and provides earlier downshifting to a lower gear, thereby improving the vehicle performance at the expense of fuel economy and driveline noise. In an "economy" setting, the traces 80 and 82 can be shifted down somewhat to provide an opposite effect.
A problem with the above-described approach is that it fails to dynamically compensate for vehicle loading. The problem is particularly apparent in truck applications, where the vehicle may be heavily loaded during a one leg of a trip, and then lightly loaded in the next leg of the trip. A shift pattern appropriate for a heavily loaded vehicle will result in excessive shift cycling in a lightly loaded vehicle, whereas a shift pattern appropriate for a lightly loaded vehicle will result in insufficient performance in a heavily loaded vehicle. Choosing a more aggressive (performance) shift pattern for heavy loads is helpful, but fuel economy may suffer unnecessarily, and it may be unrealistic to assume that the driver will choose a load-appropriate shift pattern. For this reason, controls are sometimes invoked for overriding the selected shift pattern; see, for example, the U.S. Pat. No. 5,245,893 to Koenig et al., issued on Sep. 21, 1993, and assigned to the assignee of the present invention, which overrides the upshift threshold to prevent engine over-speeding during periods of high engine acceleration, and the U.S. Pat. No. 5,172,609 to Nitz et al., issued Dec. 22, 1992, and assigned to Saturn Corporation, which reduces shift cycling by inhibiting certain upshifts based on a measure of gradeability.